Most of us expected Microsoft to drop its latest and greatest version of the last version of Windows yesterday. The highly anticipated version 1803, Redstone 4 — which many of us have been testing for weeks — looked ready to go … until it wasn’t.

Rumors are flying but, as of this writing, the actual cause for the delay isn’t public.

Microsoft, of course, has never committed to a release date. Or a build number. Or even a hokey “Spring after Fall Creators Update” style name, for that matter. (I’m still plugging for “Terry Myerson Swansong version” but doubt it’ll gain traction.)

Instead of a new version, all we got was a little patch, KB 4100375, that raised the build number from 17133.1 (which was released to the Fast ring on March 27) to 17133.73. The official blog post, co-authored by Dona Sarkar and Brandon LeBlanc, says:

UPDATE 4/10: We have released KB4100375 (OS Build 17133.73) to Windows Insiders running Build 17133 in the Fast, Slow, and Release Preview rings. This update includes the following quality improvements (no new OS features):

Which seems suitably generic, but it leaves a lot of wiggle room — and absolutely no explanation for the delay.

The download is coyly labeled “Cumulative Update for Windows 10 Version Next.”

Woody Leonhard/IDG

Zac Bowden, reporting on Windows Central, attributes the delay to a “blocking bug” — which, in normal English, just means “a bug so bad that Microsoft figured they better stop the rollout”:

According to my sources, over the weekend Microsoft found a blocking bug that, while apparently rather rare, is impactful enough to hold the release until the issue is fixed. … Thanks to feedback from Insiders in the Fast, Slow, and Release Preview rings, this bug was caught before rollout began.

Which is rather remarkable, because “Windows Insider” beta testers have had 17133.1 for two weeks — Fast ring got it on March 27, Slow ring on March 30, and Preview ring on April 10. Hard to understand how it could take two weeks to discover a bad bug.